As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. Two main approaches currently being investigated are thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR). The latter is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008).
Spin transfer (spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high. Spin transfer devices also known as spintronic devices wherein at least one of the ferromagnetic layers in a magnetoresistive (MR) junction has perpendicular magnetic anisotropy have an advantage over devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future MRAM (Magnetoresistive Random Access Memory) applications and other spintronic devices such as microwave generators.
Referring to FIG. 1, a generic MAMR writer based on perpendicular magnetic recording (PMR) is depicted. There is a main pole 1 with a sufficiently large local magnetic field to write the media bit 5 on medium 4. Magnetic flux 8 in the main pole proceeds through the air bearing surface (ABS) 6-6 and into medium bit layer 4 and soft underlayer (SUL) 7. A portion of the flux 8a returns to the write head where it is collected by write shield 2. For a typical MAMR writer, the magnetic field generated by the main pole 1 itself is not strong enough to flip the magnetization of the medium bit in order to accomplish the write process. However, writing becomes possible when assisted by a spin torque oscillator (STO) 3 positioned between the main pole and write shield 2. The STO and medium bit 5 are enlarged in FIG. 1 side (b) and the former is comprised of a high moment magnetic layer 10, and a second magnetic layer 11 that preferably has perpendicular magnetic anisotropy (PMA). Between layers 2 and 10, 10 and 11, and 11 and 1, there are nonmagnetic layers 12, 13, 14, respectively, to prevent strong magnetic coupling between adjacent magnetic layers.
Assuming a medium bit 5 with a magnetization in the direction of 9 (pointing up) is being written by a flux field 8 pointing down as in FIG. 1 side (a), part of the magnetic flux 8b goes across the gap between main pole 1 and write shield 2, and this weak magnetic field can align the magnetization of layer 11 perpendicular to the film surface from left to right. An external current source 18 creates a bias current across the main pole and write shield. The applied dc results in a current flow in a direction from the write shield through the STO 3 and into main pole 1.
Referring to FIG. 2a, the direct current generated by source 18 is spin polarized by magnetic layer 11, interacts with magnetic layer 10, and produces a spin transfer torque τs 23 on layer 10. Spin transfer torque has a value of aj m×m×mp, where aj is a parameter proportional to the current density j, m is the unit vector 15 in the direction of the instantaneous magnetization for layer 10, and mp is the unit vector 16 in the direction of magnetization in layer 11. Spin transfer torque τs 23 has a representation similar to the damping torque τD 24, and with a specific current direction, τs 23 competes with τD 24, so that the precession angle 50 is from about 0 to 10 degrees. Only when the current density is above a critical value jc will τs 23 be large enough to widely open the precession angle of magnetization 15 in layer 10 such that the oscillation has a large angle 51 usually between 60° and 160° as indicated in FIG. 2b. The large angle oscillatory magnetization of layer 10 generates a radio frequency (rf) usually with a magnitude of several to tens of GHz. This rf field interacts with the magnetization 9 of medium bit 5 and makes magnetization 9 oscillate into a precessional state 17 (FIG. 1 side b) thereby reducing the coercive field of medium bit 5 so that it can be switched by the main pole field 8.
Thus, magnetic layer 11 is often called a spin polarizer (SP) and magnetic layer 10 is referred to as the oscillation layer (OL). The aforementioned oscillation state is also achieved if main pole field 8 and medium magnetization 9 are in the opposite directions to those shown in FIG. 1. In this case, the direction of the SP magnetization 16 will be reversed, and OL as well as the medium bit will precess in the opposite direction with respect to the illustration in FIG. 1 side b.
Current MAMR technology has three main drawbacks. First, the threshold current density jc required for OL 10 to oscillate is quite high on the order of 107 to 108 A/cm2. As a result, there is a reliability concern for STO 3 since such a high current density in addition to generated heat causes electrical migration and interlayer diffusion that may damage the STO device. Secondly, the oscillation frequency of OL 10 is not easily tuned and the field across main pole 1 and write shield 2 cannot be tuned in a HDD product. Furthermore, small defects in geometry and/or material of STO layers results in an oscillation frequency shift of OL 10. If the oscillation frequency of OL 10 does not match the ferromagnetic resonance frequency of medium bit 5, the so-called microwave frequency assist will be less effective and MAMR performance will suffer. Thirdly, oscillation amplitude of OL 10 may be lower than expected due to defects and/or an extrinsic damping mechanism caused by magnetic coupling with main pole 1 or write shield 2, for example. As a result, the rf field amplitude generated by OL 10 may not be large enough to assist the recording process in some cases. Therefore, all of the aforementioned issues must be addressed in order to optimize MAMR and advance the technology to a level where it is acceptable from a manufacturing view point as well as from a performance and reliability perspective.